To an internal combustion engine mounted on a vehicle, a variable valve timing mechanism has conventionally been provided. The variable valve timing mechanism for example continuously and variably controls opening/closing timing of an intake valve by means of oil-hydraulic drive under electronic control so as to improve torque at low to medium speed, fuel economy and emission performance. The closing timing of the intake valve is set with higher priority given to startability when the internal combustion engine is started.
For example, Japanese Patent Laying-Open No. 06-323168 discloses a control apparatus for an internal combustion engine promoting atomization of injected fuel at the start of the internal combustion engine to improve startability without deterioration in emission. The control apparatus for the internal combustion engine includes: an intake valve and an exhaust valve provided in the internal combustion engine which converts reciprocating motion of a piston into rotary motion of a crankshaft to generate mechanical power, and driven by the rotation of the crankshaft to open and close an intake manifold and an exhaust manifold respectively that communicate with a combustion chamber of the internal combustion engine; a variable valve timing mechanism capable of adjusting the valve opening timing of at least the intake valve of the intake and exhaust valves; a fuel injection valve provided in the intake manifold for injecting fuel into the combustion chamber; starting state detection means for detecting the starting state of the internal combustion engine; first control means for controlling to drive the variable valve timing mechanism in the manner that, when the starting state detection means detects the starting state of the internal combustion engine, the exhaust valve is closed and the intake valve is opened after a piston reaches the top dead center, and the intake valve is closed later than a closing timing that is the closing timing after the engine is started; and second control means for controlling to drive the fuel injection valve in the manner that, when the starting state detection means detects the starting state of the internal combustion engine, the fuel is injected at the opening timing of the intake valve set by the variable valve timing mechanism.
Regarding the control apparatus for the internal combustion engine disclosed in the aforementioned document, the closing timing of the intake valve is retarded with respect to the closing timing after the start. Therefore, in the compression stroke where the piston is moved from the bottom dead center to the top dead center, the compression resistance to the piston decreases. With this decrease, the engine speed NE at the start (cranking) is readily increased so that the startability is improved.
The control apparatus for the internal combustion engine disclosed in the document retards the closing timing of the intake valve and thereby reduces the compression resistance so as to improve the startability of the internal combustion engine. However, in the case where higher priority is given to fuel economy and the valve closing timing is further retarded, a resultant problem is that an in-cylinder pressure with which the internal combustion engine can be started cannot be obtained.
It is necessary to set the intake-valve closing timing at the start of the internal combustion engine to a closing timing that does not deteriorate at least the startability at the start of the internal combustion engine. This is because the in-cylinder pressure (or compression ratio) obtained at ignition timing changes depending on the valve closing timing. Thus, in the case where higher priority is given to the startability of the internal combustion engine to set the valve closing timing, it could occur that the valve closing timing cannot be set with higher priority given to fuel economy since the variable range of the intake-valve closing timing is limited.
Further, in the case where higher priority is given to fuel economy to set the valve closing timing, the valve closing timing is further retarded so as to reduce the pumping loss. In this case, the in-cylinder pressure with which the internal combustion engine can be started could not be obtained so that the startability is deteriorated. A resultant problem is therefore that the ensured startability of the internal combustion engine and the improvement of fuel economy cannot be achieved simultaneously.